1. Field of the Invention
The present invention relates to a microwave oven, in which a transformer assembly having an improved cooling structure is provided with a temperature-sensitive switch to detect the temperature of a surface of the transformer assembly, shutting off power at an overheating temperature and, thus, preventing a transformer from becoming overheated.
2. Description of the Related Art
In general, a transformer of a microwave oven is used to boost a voltage from an external power source and apply the boosted voltage to a magnetron. The transformer is provided in an electrical components area that is partitioned from a cooking cavity, and contains various electrical and electronic parts. The transformer generates heat caused by the resistance of a coil and by generation of eddy current induced by variation of magnetic flux, so the transformer is generally air-cooled by a cooling device including a fan and a fan motor.
As illustrated in FIG. 1, in a high voltage generating unit 5 of a microwave oven equipped with an air-cooled transformer 1, a secondary side of the air-cooled transformer 1 is connected to a magnetron 3 through a voltage multiplier circuit 2.
The transformer 1 includes a primary coil L1 and secondary coils L2 and L3. The primary coil L1 provides input power (110 and 220 VAC). The voltage is boosted through electrical induction by the secondary coils L2 and L3 according to the number of turns. The heater coil L3 preheats the magnetron 3 during an early stage of operation.
The voltage multiplier circuit 2 includes a high voltage capacitor C and a high voltage diode D, and converts a voltage of about 2 KV, boosted by the transformer 1, into a high voltage of about 4 KV and applies the converted voltage to the magnetron 3.
If excessive current is applied to the high voltage generating unit 5 for an extended time, the transformer 1 overheats, and the coils may burn out. Accordingly, a high voltage fuse 4 is connected in series between the transformer 1 and the voltage multiplier circuit 2.
The high voltage fuse 4 becomes open-circuited when the high voltage capacitor C or high voltage diode D is shorted and excessive current flows. Power supplied to the magnetron 3 is shut off when the high voltage fuse 4 becomes open-circuited, so overheating of the transformer 1 is prevented.
The high voltage fuse 4 may be designed to allow electrical lines connected to connecting terminals to be severed when an excessive current of more than a certain value flows inside a protective casing of the high voltage fuse 4. The high voltage fuse 4 is connected to a secondary side, that is, a high voltage side, of the transformer 1, and must be designed to withstand high voltages. Therefore, the transformer is problematic in that the high voltage fuse 4 increases the volume of the transformer 1, it is difficult to install the transformer 1 due to the increased volume, and the manufacturing cost of the transformer 1 is increased.
Further, the high voltage fuse 4 is problematic in that it becomes open-circuited by rush current temporarily generated during normal operation, which reduces the reliability of a device using the high voltage fuse 4.
A method exists of detecting the temperature of an air-cooled transformer and shutting off power depending upon the detected temperature. Because a heat-resistant structure, which can withstand a high temperature (for example, 170° C.), must be provided on which to mount a temperature sensor on an air-cooled transformer to detect the temperature of the air-cooled transformer, this method is not effective in view of the manufacturing cost. While a temperature sensor disposed in the vicinity of a transformer reduces the requirement for a heat-resistant structure, the reliability of temperature measurement deteriorates because the temperature is measured at a position away from the transformer and is ineffective in preventing the overheating of the transformer.
Thus, while the measurement of temperature of a transformer radiating high heat is an easy and inexpensive method of preventing overheating of the transformer, it is difficult to use this method in the air-cooled transformer.